As a current type light-emitting element, Organic Light-Emitting Diode (OLED) has been increasingly applied in high performance Active Matrix Organic Light-Emitting Diode (AMOLED) display. With increasing of the size of the display, a conventional Passive Matrix Organic Light-Emitting Diode (PMOLED) display requires a shorter driving time for a single pixel, a larger transient current and thus higher power consumption. Meanwhile, a voltage drop on the nanometer indium-tin metal oxide line is too high due to the larger current application, such that operation voltage of OLED is too high and thus the operational efficiency thereof is decreased. These problems can be solved perfectly in a case in which the OLED current is inputted when switching transistors are scanned row by row in an AMOLED display.
When designing a backboard of AMOLED, a main problem to be solved is non-uniformity of luminance of OLED elements driven by various AMOLED pixel units.
At first, driving currents of light-emitting elements are provided by corresponding pixel units formed by Thin-Film Transistors (TFTs) in AMOLED. It is known that Low Temperature Poly Silicon (LTPS) TFTs or Oxide TFTs are mostly adopted. Compared with conventional amorphous-silicon TFTs, the LTPS TFTs and Oxide TFTs have higher mobility and more stable characteristics, and thus are more suitable to be applied in the AMOLED display. However, due to limitations of crystallization process, LTPS TFTs produced on a large-area glass substrate often show non-uniformity on electrical parameters such as threshold voltage, mobility and the like, and such non-uniformity may be converted to the driving current difference and luminance difference among OLED elements, that is, a mura phenomena appears, which may be perceived by human eyes. Although process of Oxide TFTs shows a better uniformity, similar to a-Si TFTs, a threshold voltage of Oxide TFT may drift under a high temperature or with a supplied voltage for a long time. Due to different display pictures, drifts of threshold voltages of TFTs in respective areas on a panel may be different from each other, which may cause display luminance difference. Such display luminance difference often renders an image sticking phenomenon since such display luminance difference has a relation to a previously displayed image.
Since the OLED light-emitting element is a element driven by a current (current-driven element), the threshold characteristic of the driving transistor in a pixel unit for driving the light-emitting element to emit light has a significant effect on the driving current and the ultimate display luminance. The threshold voltage of the driving transistor will drift under a voltage stress or light illumination, which causes the non-uniformity in the luminance of the resulted display.